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Bureau of Mines Information Circular/1988 



Geologic Conditions Affecting Coal Mine 
Ground Control in the Western 
United States 



By Gary P. Sames and Robert B. Laird 




UNITED STATES DEPARTMENT OF THE INTERIOR 





Information Circular; 91 72 



Geologic Conditions Affecting Coal Mine 
Ground Control in the Western 
United States 



By Gary P. Sames and Robert B. Laird 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 

David S. Brown, Acting Director 




Library of Congress Cataloging in Publication Data: 




^ 



V 



0^ 



Sames, Gary P. 

Geological conditions affecting coal mine ground control in the 
western United States. 



(Information circular ; 9172) 

Bibliography: p. 29-30. 

Supt. of Docs, no.: I 28.27: 9172. 

1. Ground control (Mining) 2. Coal-Geology-West (U.S.) I. Laird. Robert B. II. Title. III. 
Series: Information circular (United States. Bureau of Mines) ; 9172. 

TN295.U4 [TN288] 622 s [622'.334] 87-600353 



CONTENTS 

Page 

Abstract 1 

Introduction 

Background 

Depositional hazards 3 

Paleochannel deposits 3 

Mining hazards 4 

Field examples 7 

Crevasse splay deposits . . 11 

Mining hazards 11 

Field examples 11 

Flood basin and swamp deposits 12 

Mining hazards 13 

Field examples 14 

Other depositional hazards 14 

Lithof acies changes 14 

Coal rolls 16 

Bedding planes 16 

Fossil casts 17 

Structural hazards 20 

Faults 20 

Joints 20 

Folds 24 

Igneous dikes 24 

Clastic dikes 26 

Discussion 28 

References 29 

ILLUSTRATIONS 

1 . Map showing maximum extent of Western Interior Seaway 3 

2. Idealized sequence of paleochannel formation during peat accumulation...,. 5 

3. Three most common paleochannel types found in western U.S. coal mines 6 

4. Paleochannel that approaches and contacts coal top at high angle 8 

5. Severely slickensided shale roof adjacent to paleochannel 8 

6. Paleochannel that approaches coal top at low angle, seldom contacting it.. 9 

7. Massive supplemental roof support holding shale separated from overlying 

paleochannel 9 

8. Abandoned paleochannel within coalbed 10 

9. Area of high roof fall showing thin bedding in crevasse splay deposit 11 

10. Buckling in crevasse splay roof rock, caused by horizontal stress 12 

11. High, domed-shaped cavity in crevasse splay deposit at intersection roof 

fall 12 

12. Desiccation cracks in flood basin deposit exposed in roof 13 

13. Severely slickensided claystone roof typical of flood basin deposits 14 

14. Roof cavity caused by fall of large western U.S. kettlebottom 15 

15. Small kettlebottom showing slickedsided root structure and coalified tree 

t runk 15 

16. Moisture-sensitive claystone and roof failure between installed supports.. 16 

17. Normally flat-lying coalbed descending inferred sandstone beach ridge 17 

18. Simple bedding separation in shale i. 18 



11 



ILLUSTRATIONS— Cont inued 



Page 



19. Fossil worm burrow casts in sandstone 18 

20. Dinosaur footprint cast (sandstone) in shale 19 

21. Dinosaur trackway with many superimposed sandstone footprint casts 

separated from underlying shale by slickensides 19 

22. Portion of mine map in area of intensive normal faulting with associated 

roof falls 21 

23. Normal fault showing adjacent roof fractures and support 22 

24. Normal fault showing little roof disturbance but with evidence of pressure 

along fault plane 22 

25. Schematic of low-angle reverse faulting and failure above roof bolt 

anchorage horizon 22 

26. Joint set unrelated to cleat in coalbed 23 

27. Jointing in coalbed parallel to mining, which has caused pillar slabbing.. 23 

28. Pillar failure caused by jointing in coalbed 24 

29. Small roof fall caused by closely spaced joints 25 

30. Haulage entry in steeply pitching coalbed with updip floor rock mined for 

improved maneuverability 25 

31. Closeup of igneous dike in coal rib showing effects of heat on coal 26 

32. Several discontinuous igneous dikes cutting coalbed, with parallel 

fracturing in roof rock and coal 27 

33. Clastic dike entering coalbed from floor 27 



GEOLOGIC CONDITIONS AFFECTING COAL MINE GROUND CONTROL IN 

THE WESTERN UNITED STATES 



By Gary P. Sames' and Robert B. Laird 



ABSTRACT 

The Bureau of Mines recently initiated a study of geologic features 
that contribute to roof instability in western U.S. underground coal 
mines. The purpose of the study is to provide information for use in 
ground control planning and safety hazard reduction in that region, 
where mining activity has been increasing. Hazardous geologic condi- 
tions were surveyed in selected underground coal mines in Utah and 
Colorado, and both depositional and structural features were identified 
as potential ground hazards. 

Although the conditions found do not differ in kind from hazardous 
geologic conditions in the Eastern United States, they do differ in 
intensity of occurrence and relative importance. Three depositional 
features dominate where unstable roof occurs in western underground coal 
mines: paleochannel deposits, crevasse splay deposits, and flood basin 
deposits. Three structural features identified as hazardous, but not so 
widespread or common as the depositional hazards, are faulting, joint- 
ing, and igneous intrusions. This survey establishes a foundation for 
future studies aimed at reducing and preventing ground control accidents 
in western coal mines. 



1 Geologist, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 
2 Geologist, Goodson & Associates, Inc., Denver, CO (now with Jefferson County Plan- 
ning Department, CO) . 



INTRODUCTION 



Ground control accidents (consisting 
primarily of unexpected roof falls) are a 
major cause of fatalities in underground 
coal mining. Ongoing Bureau research 
into geologic conditions affecting coal 
mine roof stability has been concentrated 
in the Appalachian Coal Province and the 
Illinois Basin (hereafter referred to as 
the "East"). The research reported here 
is directed toward identifying the impact 
of geologic conditions found in western 
U.S. coalfields on the occurrence of 
unstable mine roof areas, and toward 
determining effective support techniques 
for their control. 

Increased demand for the low-sulfur 
coal found in the Rocky Mountain Coal- 
fields of the Western United States 
(hereafter referred to as the "West") has 
greatly increased underground coal mining 



activities in that region. The coal in- 
dustry, as a result, is contending with 
geologic conditions about which there is 
inadequate information in public domain 
mining literature. The Bureau, there- 
fore, initiated this study to identify 
and document geologic features that may 
cause roof instability in western coal 
mines. The results presented here are 
based on work conducted under Bureau con- 
tract J0145032, through Goodson & Asso- 
ciates of Denver, CO. Details of this 
work are available through the National 
Technical Information Service (NTIS) and 
as a Bureau open file report (J_) • This 
work establishes a foundation for future 
studies aimed at reducing and preventing 
ground control accidents and injuries 
in western coal mines by avoiding or con- 
trolling hazardous geologic conditions. 



BACKGROUND 



Most western coals were deposited dur- 
ing the Upper Cretaceous and Lower Ter- 
tiary Periods, at least 180 million yr 
after the Pennsylvanian-age coals of 
eastern North America. The associated 
flora found as fossilized remains in 
Cretaceous-age deposits differs from that 
of the Pennsylvanian age. Cretaceous 
coals developed from a much more advanced 
flora. The horsetails (Lepidodendron and 
Sigillaria) and clubmosses (Calamities) 
dominant during the Pennsylvanian had by 
the Cretaceous Period declined to a posi- 
tion of minor importance ( 2^ • According 
to McGookey (_3 ) , the Cretaceous climate 
was warm and humid, probably much like 
that of the southern Atlantic coast of 
the United States today. Inland areas 
were covered by pine forests, and when 
conditions were favorable, large coastal 
areas were covered by deltaic and inter- 
deltaic swamps, while the whole region 
was inhabited by dinosaurs. The swamps 
formed the extensive and sometimes very 
thick coalbeds being mined today. 



Both eastern Pennsylvanian-age and 
western Upper Cretaceous-age coals are 
products of coastal plain depositional 
environments that were strongly influ- 
enced by major transgressions and regres- 
sions (expansions and contractions) of 
sea water. However, Western Interior 
Seaway transgressive and regressive 
cycles were generally more erratic and 
rapid than those in the East during the 
Pennsylvanian. As a result, Upper Creta- 
ceous coalbeds generally exhibit greater 
lateral and vertical variability (4_) . 
Coal was deposited along the western 
shoreline as it fluctuated with the level 
of the Western Interior Seaway, a north- 
south-trending epicontinental sea that 
extended from western Utah east to the 
present-day location of the Mississippi 
River and from the Arctic Ocean to the 
Gulf of Mexico (5) (fig. 1). 

^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 




Scale, miles 



FIGURE 1.— Map showing maximum extent of Western 
Interior Seaway. 



The close of the Cretaceous Period is 
marked by complete regression of the 
Western Interior Seaway and by the begin- 
ning of the Laramide Orogeny. Structural 
deformation throughout the Rocky Mountain 
region during the Laramide Orogeny pro- 
duced a series of normal faults, creating 
a number of graben structures (depressed 
segmants of the Earth's crust), in which 
some blocks of Cretaceous and older sedi- 
ments were lowered relative to uplifted 
blocks of the same age. The graben 
structures define the intermontane basins 
where Cretaceous coalbeds are found 
today. Cretaceous sediments overlying 
the Laramide Orogenic belt that were not 
downfaulted into protected basins were 
eroded, supplying the clastic material 
for Tertiary sedimentation (3_). 

Coal deposition during the Tertiary 
Period occurred within the intermontane 
basins. Coal was deposited in freshwater 
swamps adjacent to the terrestrial flu- 
vial channels (rivers and streams) that 
transported sediments from the surround- 
ing Laramide orogenic highlands. The 
swamps were protected by levees from fre- 
quent incursions of flood waters, allow- 
ing large areas of stable peat accumu- 
lation. However, Tertiary coals are not 
currently mined underground in signifi- 
cant amounts and are generally of sub- 
bituminous or lignite rank because of 
their younger age and shallower depth of 
burial as compared with Cretaceous coal 
(6). 



DEPOSITIONAL HAZARDS 



This study indicates that paleochannel 
deposits cause the most common and severe 
roof instability problems in western coal 
mines. Crevasse splay deposits of thinly 
interbedded sandstone, siltstone, and 
muds tone, and swamp and flood basin de- 
posits of thin coals and fine-grained 
sediments also cause roof instability. 



PALEOCHANNEL DEPOSITS 

Erosion of orogenic highlands and 
transportation of sediments during depo- 
sition of western coals occurred primar- 
ily through fluvial processes. Streams 
and rivers that carried sediments through 
the coal-forming swamps are often 



^" 



recorded in the stratigraphic record and 
are visible in many underground coal 
mines as lenticular, lens-shaped deposits 
(commonly known as paleochannels) of 
sandstone or thin-bedded siltstone and 
claystone (abandoned channel fill). 

Paleochannel deposits of sandstone are 
usually massive, crossbedded, medium to 
fine grained, well indurated, and well 
sorted. The basal sand may contain rip- 
ups, channel lag deposits, floating peb- 
bles, and sole marks such as flute casts. 
Fining upward, stacked, and crossbedded 
sequences of sandstone, siltstone, and 
mudstone are commonly observed in the 
channel cross section (_7) • 

Figure 2 is an idealized sequence 
of paleochannel formation during peat 
accumulation. In figure 2.4, two active 
streams with natural levees form in a 
peat swamp. As time passes, both streams 
are affected by flooding and form 
overbank deposits of mud, clay, and silt 
(fig. 2S). In figure 2C , one stream 
channel is abandoned, infilled, and then 
buried by continued peat accumulation. 
The other stream continues to widen, is 
further affected by flooding, and devel- 
ops channel lag (coarse rock fragments 
deposited in the most swiftly flowing 
part of the stream) and slump deposits. 
As the entire area is covered by water 
during a major transgressive cycle, the 
second stream channel is infilled (fig. 
2D) . The entire sequence is then buried 
by repeated episodes of sinking down of 
the Earth's crust and transgressions and 
regressions of the sea. Lithif ication 
of the sediments and coalif ication of 
the peat by burial results in the paleo- 
channel features found in today's western 
coalbeds. 

Many hazardous depositional and cora- 
pactional conditions are associated with 
paleochannels. The three most commonly 
occurring paleochannel types found in 
western underground coal mines are de- 
picted in figure 3. Channel 4, formed 
contemporaneously with the peat and aban- 
doned, was infilled with fine-grained 
sediments and then buried under continued 
peat accumulation. Both channels B and 
C formed after peat accumulation and 
shallow burial and consist mostly of 
sandstone. Channel B cut through the 



overlying sediments and partially eroded 
the peat. Channel C partially cut 
through the overlying sediments but did 
not erode the peat. 

Mining Hazards 

Some characteristics common to paleo- 
channels found in western coal mines are 
shown in figure 3. A typical paleochan- 
nel encountered during mining can include 
the main channel body, or a series of 
small channels, which may contact and 
sometimes replace the coal; adjacent coal 
splitting; coal thickening; mud slips; 
clastic dikes; slickensided planes; frac- 
tures; distorted bedding or slump depos- 
its; and water (8). There are raining 
hazards associated with each of these 
characteristics. 

The main channel body may exhibit sev- 
eral possible hazardous conditions. The 
entire channel interface with surround- 
ing sediments is usually slickensided. 
Slickensides are highly polished, some- 
times-striated surfaces caused by dif- 
ferential compaction and uneven loading 
(9)* The slickensided channel boundary 
envelope constitutes a surface of weak- 
ness or lack of bonding, allowing the 
rock directly beneath and adjacent to the 
channel to separate as coal is mined. 
Coal streaks within the channel sandstone 
can also create planes of weakness that 
may lead to separation after mining. 
Jointing sometimes occurs in paleochannel 
sandstone and can result in blocklike 
failure when combined with bed separa- 
tion. Other characteristics that are 
common to sandstone paleochannels but 
do not usually create roof fall hazards 
are channel lag and cross-bedding, the 
arrangement of minor beds or laminations 
at an angle to the main stratified unit 
(10). Cross beds with abundant mica or 
coal streaks can create delamination 
hazards. 

Horizontal splitting of coalbeds by 
claystone partings in western coal mines 
can signify the proximity of a contem- 
poraneous paleochannel. A coalbed split 
usually begins as a thin mud or clay 
parting, the result of sheet flood depo- 
sition of very fine sediment, several 
hundred feet from the paleochannel. 



Cloy parting 



Clay parting 




Shale 
1 Sandstone 

Crevasse splay 
Coal 

Claystone 
Shale 



Crevasse splay 
Peat 
Clay 
Mud 



Crevasse splay silt 
and mud 



Overbank mud" 
and clay 



FIGURE 2.— Idealized sequence of paleochannel formation during peat accumulation. A, Two 
active streams with natural levees form in Cretaceous peat swamp; B, both streams are affected 
by flooding and form overbank deposits as time passes; C, one stream channel is abandoned 
as the other continues to develop; D, sequence is completed by marine transgression and burial. 




KEY 

Bedded sandstone 

Cross bedded sandstone 

Siltstone 

Sandy shale 

Shale 



^Clay 

MB Coal 

liiiiill Fractures 

j^^j Slips and slickensides 

l^ 5 *! Mud slips 



Not to scale 



FIGURE 3.— Three most common paleochannel types found in western U.S. coal mines. A, Paleochannel formed 
contemporaneously with peat; B, paleochannel, formed after peat accumulation, that partially eroded the peat; C, 
paleochannel, formed after peat accumulation, that did not erode overlying sediments to the peat. 



As mining progress toward the channel, 
the parting can thicken and separate the 
coalbed into two seams. If the split 
becomes uneconomical to mine, conditions 
in the roof can resemble those in figure 
3 A. 

An increase in coal thickness may be 
noted when mining near a paleochannel. 
Differential compaction or uneven load- 
ing, which squeezes the soft peat from 
beneath the channel and swells it to 
either side, is commonly believed respon- 
sible for increases in adjacent coal 
thickness (11 ). No hazardous conditions 
are generally associated directly with 
increased coal thickness, but the in- 
crease indicates other disturbances in 
the surrounding strata. 

"Mud slip" is a mining term used to 
describe a claystone or raudstone lens in 
the roof adjacent to a paleochannel (_8) 
(fig. 3B-C) . Mud slips are usually found 
on the inside meanders of paleochannels 



and are commonly slickensided on both 
sides. Mud slips adjacent to paleochan- 
nels can cover a large area, depending on 
the size of the channel and meander bend. 
Mud slips can approach 30 ft along strike 
and can be 10 to 15 ft wide and several 
feet thick. However, most often mud 
slips are 3 to 4 ft in diameter and 2 to 
5 in thick. When grouped together as in 
figure 3B and 3C, mud slips can create a 
large area that is subject to roof fall. 

Clastic dikes adjacent to paleochannel 
deposits are compactional injections of 
soft sediment into ruptures in the coal 
during or after the coalif ication process 
(JL_2 ) . Clastic dikes occur both parallel 
and perpendicular to paleochannel depos- 
its in a linear to curvilinear trend 
(13). The more common structural occur- 
rences of clastic dikes and their effects 
on the roof are discussed more fully in a 
later section. 



Abundant slickensides occur adjacent 
to paleochannels, caused by differential 
compaction of sand and adjacent finer 
grained sediments. Because they have 
very little cohesive strength, slicken- 
sides are planes of weakness that facili- 
tate roof failure. They are associated 
with all three paleochannel types shown 
in figure 3. In their most common form, 
slickensides adjacent to paleochannels 
occur in two basic orientations: paral- 
lel and perpendicular to the main chan- 
nel body (fig. 35). These orientations 
create wedges of roof prone to failure 
at their slickensided margins, creating 
falls that are commonly known as "horse- 
backs" by miners. 

The frequency of fractures in the roof 
can increase as a paleochannel is ap- 
proached (8, _14_)» Fractures are often 
visible in the roof as "goatsbeard," a 
mining term used to describe the crystal- 
lization of gypsum or anhydrite from 
water seepage. The fractures, shown 
associated with all three paleochannel 
types in figure 3, can cause blocklike 
failure of the roof. 

Distorted or slump bedding sometimes 
occurs adjacent to paleochannels. Dis- 
torted bedding is the result of differ- 
ential compaction of sediments. Slump 
bedding is evidenced by discrete blocks 
of rock with rotated bedding relative 
to the surrounding rock. Slump bedding 
is the result of mass movement through 
rotational sliding of the channel bank. 
Slickensides and fractures associated 
with both distorted and slumped bedding 
cause mining hazards and change the roof 
bolt anchorage characteristics of the 
roof. 

Paleochannels are often the source 
of water migration into the mine (ji). 
Adverse conditions caused by water seep- 
age from paleochannels include weaken- 
ing and deterioration of roof rock, 
weathering of joints and fractures, and 



lubrication of slickensides. The pres- 
ence of excessive moisture tends to in- 
tensify the effects of any other adverse 
conditions already present. 

Field Example s 

Figure 4 shows a typical paleochannel 
in western coal mines, which approaches 
and contacts the top of the coalbed 
at a high angle. This situation illus- 
trates the difficulty in supporting 
finer grained sediments (in this case a 
dark gray shale) adjacent to a sandstone 
paleochannel. The roof adjacent to the 
channel progressively failed because of 
slickensides both within the shale and at 
the channel boundary. The roof bolts 
nearest the rib were anchored in the 
channel sandstone, but the slickensided 
shale around them failed. The bolts 
installed in the remaining width of the 
entry in figure 4 were anchored within 
the shale and fell with it. Although 
this roof failed progressively, the 
potential for a sudden, massive fall was 
present. The beginning of roof rock 
failure between closely spaced supports 
in severely slickensided shale adjacent 
to a sandstone paleochannel is shown in 
figure 5. 

Figure 6 shows a typical paleochannel 
approaching a coalbed at a low angle, 
seldom contacting it. The hard silty 
shale underlying the massive channel 
sandstone is highly fractured. The shale 
separated from the sandstone, falling 
between the bolts, and breaking the metal 
straps. Supplemental support (steel 
straps, wood posts, and crossbars) were 
installed in the main haulage entry under 
the channel to control the shale. Sepa- 
ration from the sandstone still occurred, 
but the supplemental support maintained 
the roof for the remaining width of the 
channel (fig. 7). 




Roof rubble 

nHBHBBHH 

FIGURE 4.— Paleochannel that approaches and contacts coal top at high angle. Note adjacent shale roof failure. 




Shckensided shale roof 



FIGURE 5.— Severely shckensided shale roof adjacent to paleochannel. 



Sandstone channel 





Coal 







FIGURE 6.— Paleochannel that approaches coal top at low angle, seldom contacting it, with fractured and unstable 
underlying shale. 




FIGURE 7.— Massive supplemental roof support holding shale separated from overlying paleochannel. 



10 



Figure 8 shows an abandoned channel 
that was infilled, then buried by 
continued peat accumulation (similar to 
that in figure 3A) . This channel is 
exposed in a crosscut by a roof fall 
that included the overlying rider coal. 
A finding-upward sequence is apparent, 
grading from a massive, well-sorted sand- 
stone, to siltstone, then mudstone, and 
back into coal. The sandstone is jointed 



and weakened by carbonaceous bedding lam- 
inations. The rider coal presents the 
most hazardous condition in this situ- 
ation because of poor bonding between it 
and the overlying and underlying strata. 
Anchorage of the roof bolts below the 
rider contributed directly to this roof 
fall, which included the top of the rider 
coal. 




FIGURE 8.— Abandoned paleochannel within coaibed. 



11 



CREVASSE SPLAY DEPOSITS 



Field Examples 



Crevasse splay deposits of interbedded 
sandstone, siltstone, and mudstone adja- 
cent to paleochannel trends are common 
in the West. Rivers at flood levels 
scour channels or crevasses through the 
levees. Lobes of sands and fines are 
then deposited where the flood water 
spreads into the adjacent basin. The 
sand is of the same composition as the 
main channel, but is often restricted in 
thickness, resulting in thin sand sheets 
that are dispersed in a lobate pattern. 
Once indurated, the sandstone sheets are 
separated, in a vertical sequence, by 
thin siltstone and mudstone laminations 
(15-16). 



Figure 9 shows a crevasse splay deposit 
exposed in a high roof fall. The thinly 
laminated, brittle nature of the deposit 




Mining Hazards 

Crevasse splay deposits occur above the 
coalbed in most mines in the West. Cre- 
vasse splays contribute most signifi- 
cantly to mine roof instability when they 
are within 6 to 10 ft of the top of the 
coalbed and therefore constitute a part 
of the immediate roof (17). The thin, 
incohesive nature of crevasse splay bed- 
ding commonly causes delamination of the 
roof. Massive falls extending above the 
anchorage horizon sometimes result from 
this condition. 

Observations made in this study indi- 
cate that crevasse splay roof becomes 
less stable with increasing overburden 
thickness. In mines where the overburden 
exceeded approximately 1,500 ft, roof of 
thinly laminated sandstone, siltstone, 
and mudstone became highly unstable. 
This effect is especially evident in in- 
tersections. The thin, brittle lamina- 
tions of the crevasse splay appear to 
deform and fracture under the load, some- 
times resulting in large and severe roof 
falls. 



> 




Crevasse splay bedding 




FIGURE 9.— Area of high roof fall showing thin bedding in 
crevasse splay deposit. Note evidence of pressure deformation. 



12 




FIGURE 10. — Buckling in crevasse splay roof rock, caused by horizontal stress. 



is evident. Figure 10 shows failure be- 
lieved to be induced by high horizontal 
pressures. The roof strata have buckled, 
requiring additional support. Control of 
this type of failure is difficult. The 
roof failure effectively interrupts the 
continuity of the roof span and will 
eventually require more artificial sup- 
port. Figure 11 illustrates another com- 
mon roof fall in crevasse splay deposits: 
a high, dome-shaped fall. Although this 
may be an extreme example (the photograph 
was taken above the coal on top of a 
canopied support), it shows the bedded 
nature of crevasse splays and the scale 
of roof fall problems they can cause. 

FLOOD BASIN AND SWAMP DEPOSITS 

Flood basin deposits are generally 
fine-grained sands, clays, and silts. 
They are interlaminated, cross laminated, 
and often contain desiccation cracks 
(fig. 12). Flood basin deposits are of- 
ten burrowed and rooted. Under suitable 
conditions, peat is deposited in flood 
basin swamps (7). Flood basin deposits 




FIGURE 11.— High, domed-shaped cavity in crevasse splay 
deposit at intersection roof fall. 



13 




FIGURE 12.— Desiccation cracks in a flood basin deposit exposed in roof. 



commonly form shale, clays tone, and car- 
bonaceous mudstone roof in western coal 
mines (18). The shales often grade 
upward into overlying swamp deposits of 
claystone, rooted mudstones, and thin 
coals or rider seams. 

Mining Hazards 

The finest grained flood basin depos- 
its, claystones and mudstones, generally 
contain the features that cause the most 
common problems in this type of roof: 
slickensides, "kettlebottoms ," rider 
seams, and fissile bedding. The sensi- 
tivity of most claystones to moisture 
adds to the roof support problems. 

Slickensides, described previously, are 
usually more prevalent in mudstone and 
claystone. In areas of claystone immedi- 
ate roof, many small slickensided planes 
can weaken the whole rock fabric, making 
support difficult, as the rock falls 
between roof bolts (19). 



Kettlebottoms in western coal mines are 
quite different from those that commonly 
occur in the East. An eastern kettle- 
bottom is defined as a columnar mass of 
rock (the preserved cast of an ancient 
tree stump) embedded in the normal coal 
mine roof strata and separated from them 
by a surrounding coal ring and slicken- 
sides (20). A western kettlebottom, on 
the other hand, is more a result of the 
tree root structure than of the trunk 
itself. The root systems of the trees 
that grew in the Cretaceous swamps of the 
West resulted in slickensides that radi- 
ate from the base of the tree. There- 
fore, kettlebottoms in the West fail at 
the slickensided boundaries of the struc- 
ture in a concave upward circle, forming 
an inverted, funnel-like cavity in the 
roof. 

Rider seams are common in swamp-type 
deposits. A rider seam is typically a 
thin, discontinuous coalbed above the 
coalbed being mined. When the interval 



14 



between the rider seam and main coal- 
bed thins, the rider forms a plane of 
weakness above the roof-bolting horizon, 
which can result in massive falls (21). 

Deterioration of moisture-sensitive 
claystone and shale is usually a time- 
dependent phenomenon, with slow slaking 
of the roof during the dry winter sea- 
son, and quickening deterioration during 
the more humid summer conditions (22) . 
Deterioration caused by moisture can lead 
to a loss of mechanical roof bolt ten- 
sion. The resulting roof failure by 
attrition increases debris on the mine 
floor, resistance to ventilation airflow, 
and in extreme cases, pillar height, with 
resultant pillar instability. 

Some f ossilif erous shales associated 
with swamp deposits are highly fissile. 
Shales of this type often split away from 
the roof between installed support, cre- 
ating scattered, small roof rockfall haz- 
ards from hanging slabs. 

Field Examples 

Figure 13 shows an example of severely 
slickensided claystone roof. The many 
random planes of weakness make support 
difficult, and during periods of high 
humidity, even more rapid deterioration 
from slaking will occur. 

Figure 14 shows a roof fall cavity 
caused by a very large western kettlebot- 
tom. This kettlebottom was approximately 
16 ft in diameter, but others ranged down 
to 10 in. in diameter (fig. 15). Western 
kettlebottoms are difficult to distin- 
guish from the normal mine roof in many 
cases. The only indication of their 
presence is often a slickenside that may 
be indistinguishable from many others. 
However, when a kettlebottom is identi- 
fied, others should be suspected in the 
vicinity. 

Deterioration of moisture-sensitive 
shale and claystone roof is a well-docu- 
mented phenomenon in eastern coal mines. 
Figure 16 shows an example of this prob- 
lem in a western mine. Moisture deteri- 
oration of the claystone roof resulted in 
progressive failure between the installed 
supports, up to a higher, more resistant 
strata. As is evident, this type of 




FIGURE 13. — Severely slickensided claystone roof typical of 
flood basin deposits. 

failure creates obstructions in haulage 
and ventilation entries and requires con- 
stant maintenance. 

OTHER DEPOSITIONAL HAZARDS 

Other, less common, but often severe 
instability problems not necessarily re- 
lated to any one depositional condition 
are caused by lithofacies changes, coal 
rolls, bedding planes, underclays, and 
dinosaur footprint casts. 

Lithofacies Changes 

A particularly hazardous type of west- 
ern coal mine roof is characterized by 
interf ingering lithofacies changes (adja- 
cent deposits of differing rock types). 
In general, lithofacies changes occur 
in areas of a delta plain that were 
periodically inundated or exposed by 
transgressing and regressing marine 



15 




FIGURE 14.— Roof cavity caused by fall of large western U.S. kettlebottom. 



Coalified trunk 





FIGURE 15.— Small kettlebottom showing slickensided root structure and coalified tree trunk. 



16 




FIGURE 16.— Moisture-sensitive claystone roof failure 
between installed supports. 

waters, resulting in fine-grained sedi- 
ments being deposited adjacent to coarse- 
grained ones. Crevasse splay deposits 
and shifting distributary channels also 
result in this depositional condition 

ui, 20. 

Examples of the hazards caused by lith- 
ofacies changes are presented in the 
sandstone channel illustrations (figs. 
2-7), where two differing rock types 
deposited adjacent to one another have 
differing rock properties. As in the 
sandstone channel example (fig. 2), if 
one unit compacts more than the other, 
slickensides , distorted bedding, and some 
fracturing will likely occur in the more 
compactible unit. Also, a less compac- 
tible rock unit, such as a sandstone lens 
surrounded by shale, will have compac- 
tional deformation on all sides (23-24). 



In general, wherever the roof rock type 
in a mine changes abruptly, the presence 
of adverse geologic structure and chang- 
ing roof bolt anchorage characteristics 
should be suspected. 

Coal Rolls 

In western underground coal mines, a 
coal roll, as defined by Bunnell (25) , is 
"an undulation in the coal seam." The 
coalbed undulates and may thin in re- 
sponse to local thickening in the floor. 
Bunnell offers as a likely explanation 
for coal roll formation the imbricately 
structured beach ridges that are produced 
by high-energy waves. Peat deposits 
superimposed on the beach ridges result 
in the formation of a coalbed that is 
draped over the beach ridges. 

Figure 17 shows an entry in a mine that 
operates in a coalbed with a gentle 
regional dip. In a local area of this 
mine, the coalbed is alternately rising 
and descending over inferred beach 
ridges. In figure 17, the coalbed is 
believed to be descending the steep face 
of a sandstone beach ridge. Production 
problems are posed by the abrupt, steep 
grades, which make movement of machinery 
more hazardous and difficult. Water of- 
ten accumulates in the troughs, adding to 
haulage problems. Previously described 
differential compaction conditions can 
occur at the apex of the beach ridges. 

Bedding Planes 

A common adverse geologic condition in 
western coal mines is weak bonding along 
bedding planes within the roof rock. As 
has often been documented, a relatively 
small piece of rock delaminating from the 
roof can prove dangerous. Bedding planes 
represent planes of weakness caused 
either by two periods of deposition of 
like sediments separated by a time lag or 
by deposition of two different sediment 
types (10). Bedding in the common roof 
rock types can range from thick-bedded 
sandstone to thinly bedded crevasse-type 
deposits to extremely fissile fossilifer- 
ous shales. 



17 




FIGURE 17.— Normally flat-lying coalbed descending inferred sandstone beach ridge. 



The most hazardous occurrences of this 
type are in the previously discussed cre- 
vasse splay deposits. Fissile bedding in 
f ossiliferous shales is also hazardous, 
with many loose overhanging slabs common 
in this roof type. Constant attention is 
required to keep these loosely hanging 
slabs scaled or supported. Figure 18 
shows an example of bedding separation in 
shale. 

Fossil Casts 

Two types of fossil casts were observed 
in western mines: worm burrows and dino- 
saur footprints. Worm burrows, trace 
fossils of a burrowing marine worm, are 
more a curiosity than a mining hazard. 
Figure 19 shows a group of burrows ob- 
served beneath the silicified bark of a 
tree branch or trunk. Dinosaur footprint 
casts, however, are more common. Most 
dinosaur footprints consist of three 
forward toes with one rear "thumb" (-26) 
(fig. 20), although four-toed footprints 



have been observed. Footprint trace fos- 
sils developed where dinosaurs crossed 
the spongy, uncompacted peat of the 
swamp. The weight of the animals caused 
the peat to compact. To the extent that 
the peat was unable to rebound, a depres- 
sion was formed that could later be in- 
filled by other sediments, commonly sand 
(27). Because the sandstone of the foot- 
prints is usually part of an overlying 
sandstone unit, dinosaur footprint casts 
are not easily separated from the roof. 
However, in rare cases, they do separate 
from the roof and fall. Also, the under- 
side of the footprint cast is usually a 
slickensided surface that allows any un- 
derlying rock to fall. Figure 21 shows 
an area of repeated dinosaur movement 
with many footprints superimposed. The 
footprints can be distinguished by their 
bulging downward surfaces. The 1-ft- 
thick fossilif erous shale immediately 
above the coal separated and fell from 
the slickensided footprints. 



18 




FIGURE 18.— Simple bedding separation in shale. 




FIGURE 19.— Fossil worm burrow casts in sandstone immediately overlying a coalbed. 



19 




FIGURE 20.— Dinosaur footprint cast (sandstone) in shale. ' Note the characteristic three forward toes and one rear thumb. 




; .ltf| 






w 

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v^ 



Dinosaur footprints 




FIGURE 21.— Dinosaur trackway with many superimposed sandstone footprint casts separated from the underlying shale 
by slickensides. 



20 



STRUCTURAL HAZARDS 



Hazardous structural conditions are not 
as common as depositional hazards in 
western underground coal mines. The most 
commonly encountered structural hazards 
are faults and joints. Folds, while not 
as common as faults or joints, can also 
present serious mining difficulties. Two 
other minor structural hazards that 
should be recognized are igneous intru- 
sions and clastic dikes. 

FAULTS 

Faults are fractures or fracture zones 
in the rock strata along which there has 
been displacement of the two sides rela- 
tive to one another and parallel to the 
fracture(s) (10). Faults are classified 
by the direction and angle of their move- 
ment. Several types of faults (normal, 
reverse, and strike-slip being the most 
common) occur in western coalfields with 
varying amounts of displacement. How- 
ever, normal faulting, with the down- 
thrown block on the downdip side of the 
fault, was the predominant type encoun- 
tered during this study. 

Minor to severe ground control problems 
are associated with faults in western 
coal mines. Many faults observed during 
this study had displacements of only sev- 
eral inches and caused only minor ground 
control problems. In another instance, a 
fault zone 120 ft wide, with many closely 
spaced fault planes, vertically displaced 
the coal 18 to 30 ft and required rock 
tunnel development through severely 
broken roof to reach adjacent reserves. 

Figure 22 shows a portion of a mine map 
in an area of intensive normal faulting. 
These faults all follow a distinct trend, 
and are associated with many roof falls, 
as indicated on the map. Displacement 
along most of these faults is between 3 
and 5 ft, with several faults splaying 
into two or more distinct fault planes. 
Figure 23 shows one of the faults mapped 
in figure 22. This fault is nearly ver- 
tical, with approximately 3.5 ft of dis- 
placement. Although roof disturbance is 
minimal in this area, additional support 
is necessary on each side of the fault 
because of increased fracturing of the 
roof. 



Figure 24 shows a less steeply inclined 
fault with approximately 1 ft of dis- 
placement. There is little of the usual- 
ly associated adjacent roof disturbance, 
but the difficulty in penetrating the 
fault zone with a roof bolt drill indi- 
cates that there is active pressure be- 
ing exerted along the fault. Two drill 
steels that were bound in the roof by the 
fault are visible in the figure. 

Another less common, but possibly more 
hazardous fault type that occurs in west- 
ern coal mines is the low-angle reverse 
fault. Figure 25 depicts this type of 
occurrence in a mine, with an associated 
roof fall. The low angle of the fault 
plane keeps it subparallel to the coal- 
bed for an extended distance into the 
roof. In this example, as is common with 
this type of fault, the roof failed af- 
ter the fault extended above the roof 
bolt anchorage horizon, but failure can 
also occur before installation of perma- 
nent support. Low-angle reverse faults 
generally cause more disturbance in the 
surrounding roof rock than do normal 
faults. 

JOINTS 

Joints are divisional planes or sur- 
faces in the rock strata along which 
there has been no visible movement paral- 
lel to the plane or surface (10). Joint- 
ing in western coal mines affects both 
rib and roof control. The effects of 
jointing vary with joint patterns, joint 
density, and the lithology of the af- 
fected strata. 

Prominent jointing within the coalbed 
that is unrelated to original cleat some- 
times occurs in western coal mines (28) . 
Figure 26 shows a joint set in a coalbed. 
Prominent jointing in the coalbed, when 
parallel to the direction of mining, can 
cause pillar slabbing that results in 
widening of the mine opening, creating a 
larger span to support (fig. 27). Joint- 
ing in the coalbed at an angle to mining 
is less detrimental to ground control but 
can still result in pillar failure at the 
corners and the need for additional roof 
support (fig. 28). 



21 



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FIGURE 22.— Portion of mine map in area of intensive normal faulting with associated roof falls. 



22 




FIGURE 23.— Normal fault (with 3.5 ft of vertical displacement) showing adjacent roof fractures and support. 




FIGURE 24.— Normal fault (with approximately 1 ft of vertical 
displacement) showing little roof disturbance but with evidence 
of pressure along fault plane. 




KEY 



I I Shale r.".""l Sandy shale 

IV. ■/:] Sandstone b--s-~i Underclay L - 



10 

_ 



■=s. Direction of Er— :1 Mudstone 
faulting 



Scale, ft 



FIGURE 25. — Schematic of low-angle reverse faulting and 
failure above roof bolt anchorage horizon. 



23 




Joints 




FIGURE 26.— Joint set unrelated to cleat in coalbed. 




FIGURE 27.— Jointing in coalbed parallel to mining, which has caused pillar slabbing. 



24 




FIGURE 28.— Pillar failure caused by jointing in coalbed. 



Jointing in the roof of underground 
coal mines in the West also contributes 
to roof instability, probably to a much 
greater extent than in the East (23). 
Joints tend to separate the roof into 
blocks. In roof rock with incohesive 
bedding, this condition can lead to fail- 
ure of slabs of rock between support. An 
example of joint-caused roof failure is 
given in figure 29. This area of closely 
spaced joints in the roof was heavily 
supported with both roof bolts and steel 
straps. Nonetheless, roof failure oc- 
curred between the supports along joint 
and bedding planes. 

FOLDS 

A fold is flexure of the rock strata. 
In western coal fields, folding is gen- 
tle to moderate and locally variable. 
Where folding is gentle, a coalbed may 
be slightly inclined, and the effect on 
ground control will be minimal. However, 
several minable coalbeds in the West are 
folded more tightly, presenting steeply 
pitching mining conditions. 



Roof control problems associated with 
folding include fractures, joints, and 
faults. Jointing can be extensive in the 
fold crest. Lateral rock movement was 
noted in the flanks of folds, which, com- 
bined with jointing, can cause severe 
roof instability. 

Figure 30 shows a mine working down 
a fold limb. In this steeply pitching 
coalbed, movement of workers and machin- 
ery is difficult. In this mine, the up- 
dip floor rock in main entries is being 
removed to facilitate transportation and 
haulage. 

IGNEOUS DIKES 

Igneous dikes are a fairly common oc- 
currence in western coal mines. However, 
ground control problems associated with 
them are usually not severe. Igneous 
dikes are usually found in preexisting 
fault and joint systems (29). The mine 
map in figure 22 is a good example of 
this. Several igneous dikes in this por- 
tion of the mine are intruded parallel 
to the normal faults, presumably along 



25 




FIGURE 29.— Small roof fall caused by closely spaced joints. 




FIGURE 30.— Haulage entry in steeply pitching coalbed with updip floor rock mined for improved maneuverability. 



26 



preexisting planes of weakness. Usually, 
the high heat flow associated with dikes 
has created natural coke in the surround- 
ing coal. Figure 31 is a closeup photo- 
graph of a dike in a coal rib. The adja- 
cent coal has been converted to coke and 
exhibits the columnar jointing that is 
common in slowly cooled rock. Figure 32 
shows several parallel igneous dikes 
intruded into a severely fractured area 
of a mine. The fractures in this area 
of the mine are probably the result of 
the same forces that produced the normal 
faulting in other areas, and the unstable 
roof is due more to the fractures than 
to the igneous dike. Even though igneous 
dikes are a common occurrence, no coal 
mines reported any major ground control 
problems associated with them. They are 
more a nuisance, creating heavy wear on 
mining equipment and slowing production. 

CLASTIC DIKES 

Clastic dikes are intrusive sedimentary 
features in western coal seams that tran- 
sect the coal from either above or below. 



*£?■ 




Most clastic dikes in western coal mines 
are of a sandstone texture and can enter 
the coalbed from either the roof or the 
floor (30). Figure 33 shows a clastic 
dike that entered the coalbed from below 
and failed to completely transect it. 
The clastic dikes observed averaged 2 to 
8 in wide, although widths of up to 3 ft 
were reported. The length of clastic 
dikes also varies , but dikes have been 
reported from less than 3 ft to more than 
3,000 ft long. 

Clastic dikes in western coal mines are 
not regarded as especially detrimental to 
ground conditions. In contrast, in east- 
ern coal mines, the weakening effect of 
clastic dikes extends from 3 to 12 ft 
above the top of the coalbed. The dikes 
are generally wedge-shaped masses of 
slickensided claystone or mudstone fill- 
ing crevices in the coalbed. In addition 
to forming a discontinuity in the coalbed 
and immediate roof, eastern clastic dikes 
and surrounding rocks are usually heavily 
slickensided and, therefore, likely to 
break away from the roof in thick blocks 
as the supporting coalbed is mined (17). 








V 






FIGURE 31.— Closeup of igneous dike in coal rib showing effects of heat on coal. 



27 








FIGURE 32.— Several discontinuous igneous dikes cutting coalbed, with parallel fracturing in roof rock and coal. 




FIGURE 33.— Clastic dike entering coalbed from floor. 



28 



Although some reports of clastic dikes 
causing hazardous roof do occur in the 
West, most clastic dikes seem to be asso- 
ciated with strong sandstone roof. As 
is the case with igneous dikes, western 



clastic dikes are more of a nuisance, 
causing heavy wear on mining equipment 
and slowing production. In addition, 
they can, in gassy mining conditions, 
create an ignition source. 



DISCUSSION 



This geologic study for underground 
coal mine ground control in the Western 
United States shows that roof stability 
is significantly affected by both depo- 
sitional and structural geologic condi- 
tions. The hazardous geologic features 
do not differ in kind from those commonly 
found in the East, but they do differ in 
intensity of occurrence. 

Three depositional roof types were 
found to dominate in unstable roof: pal- 
eochannel deposits, which cause the most 
common and severest roof instability; 
crevasse splay deposits of thinly inter- 
bedded sandstone, siltstone, and mud- 
stone; and swamp and flood basin deposits 
of thin coals and fine-grained sediments. 

Mining hazards associated with paleo- 
channels are slickensided roof rock mar- 
gins, fractures, mud slips, clastic 
dikes, coal splitting, distorted bedding 
or slump deposits, and water. Hazards 
associated with crevasse splays are de- 
lamination of the roof and failure above 
the roof bolt anchorage horizon. Also, 
increased instability was observed in 
crevasse splay roof as overburden thick- 
ness increased. Flood basin deposits 
contain many common mining hazards, 
including slickensides , kettlebottoms , 
rider seams, deterioration from moisture, 
and fissile bedding. 

Hazardous structural geologic condi- 
tions occurring in western underground 
coal mines are not so widespread or 
common as the identified depositional 
hazards. The most commonly encountered 
structural hazards are faults and joints. 
Folds, while not as common as faults or 
joints, can also present serious mining 



difficulties. Two other structural haz- 
ards that cause fewer ground control 
problems, but are often encountered, are 
igneous and clastic dikes. 

Minor to severe ground control problems 
are associated with faults. The coalbed 
can be completely offset, requiring rock 
tunneling through fractured rock to re- 
enter the mining horizon. More commonly, 
the offsets are small, normal displace- 
ments that fracture the surrounding roof 
rock. Joints in the roof strata break 
the roof span, making support difficult. 
Joints in the coalbed cause rib instabil- 
ity that can lead to roof failure. Fold- 
ing of the strata causes pitching seam 
conditions, lateral rock movement in the 
roof, and difficulty in movement of work- 
ers and machinery. Igneous dikes, while 
not always causing severe roof condi- 
tions, can weaken the roof with associ- 
ated fracturing. Clastic dikes also have 
associated fracturing and slickensides 
that weaken the roof and make ground sup- 
port difficult. 

This report describes the initial phase 
of a program intended to provide back- 
ground geologic information on western 
coalfields, to define and describe the 
geologic features and adverse conditions 
that create ground control problems, and 
to characterize the behavior of these 
features in response to mining. While 
the scope of this report is limited to a 
preliminary survey, it provides guidance 
for further study into the prediction of 
these geologic hazards, and the develop- 
ment of roof support requirements for 
effective ground control in mines. 



REFERENCES 



29 



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